Chemistry · Coordination Compounds · NEET
In a free metal ion all 5 d-orbitals have the SAME energy (degenerate). When 6 ligands (lone pairs = negative charge) come close in an octahedral shape, they push on the metal's d-electrons. But they do not push all 5 orbitals equally. The orbitals whose lobes point STRAIGHT at the ligands feel more repulsion and rise in energy; the orbitals that point BETWEEN the ligands feel less repulsion and drop. So the single energy level splits into two levels. This splitting is the whole idea of Crystal Field Theory.
In an octahedral field the ligands come along the x, y, z axes. Two orbitals point along the axes: d(z^2) and d(x^2-y^2). These point RIGHT at the ligands, so they are repelled the most and go UP — this upper pair is called eg. The other three, dxy, dyz, dzx, point in between the axes, feel less push, and go DOWN — this lower set of three is called t2g. Quick trick: any d-orbital with 'x^2', 'y^2' or 'z^2' in its name lies on an axis, so it is in the higher eg set.
In a tetrahedral complex the 4 ligands sit at alternate corners of a cube, so they do NOT lie on the x, y, z axes — they point between the axes. Now the orbitals pointing between axes (dxy, dyz, dzx) are closer to the ligands and get repelled MORE, so they go UP (called t2). The axis-pointing orbitals d(z^2) and d(x^2-y^2) point between the ligands and go DOWN (called e). So the pattern is flipped compared to octahedral. Note: in tetrahedral we drop the 'g' (no centre of symmetry), so it is e and t2, not eg and t2g.
Two reasons make the tetrahedral gap small. First, a tetrahedral complex has only 4 ligands instead of 6, so there is less total repulsion. Second, NONE of the d-orbitals point directly at the ligands in a tetrahedron, so even the higher set is not pushed very hard. Together these give the standard result Δt = (4/9) Δo ≈ 0.44 Δo, for the same metal and ligands. NEET expects you to just use this ratio in numericals.
Almost always HIGH spin. Δt is so tiny (only 4/9 of Δo) that it is nearly always smaller than the pairing energy. So electrons prefer to jump to the upper t2 orbitals rather than pair up in the lower e orbitals. That is why you can basically assume every tetrahedral complex is high spin — low-spin tetrahedral complexes are extremely rare and not asked in NEET. The strong-field / weak-field, high-spin / low-spin decision really matters for octahedral complexes.
Δo depends on the ligand and the metal. Stronger-field ligands (higher in the spectrochemical series, e.g. CN- , CO, en, NH3) give a LARGER Δo; weak-field ligands (F-, Cl-, H2O) give a smaller Δo. A higher oxidation state of the metal and metals from the 4d/5d series also increase Δo. Since colour comes from a d-d jump across this gap, a bigger Δo means higher energy absorbed and a shorter wavelength of light absorbed.
The Crystal Field Stabilisation Energy (CFSE) for [CoCl6]4- is 18000 cm^-1. The CFSE for [CoCl4]2- will be:
What is the correct electronic configuration of the central atom in K4[Fe(CN)6] based on crystal field theory?
Statement I: Both [Co(NH3)6]3+ and [CoF6]3- are octahedral but differ in magnetic behaviour. Statement II: [Co(NH3)6]3+ is diamagnetic whereas [CoF6]3- is paramagnetic. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Δo is the energy gap between the lower t2g set and the upper eg set of d-orbitals in an octahedral complex. It is called the crystal field splitting energy. A bigger Δo means a stronger field and can force electrons to pair up (low spin).
No. Tetrahedral complexes have no centre of symmetry, so the 'g' is dropped. The two sets are called e (lower, 2 orbitals) and t2 (upper, 3 orbitals) — the reverse order of octahedral.
Yes. For the same metal and ligand, Δt = (4/9)Δo ≈ 0.44 Δo. NEET repeatedly asks a direct numerical using this exact ratio, so it is a must-remember.
Because Δt is very small (only 4/9 of Δo), it is nearly always less than the electron pairing energy. So electrons prefer to occupy the upper t2 orbitals singly rather than pair up, giving high spin.
An electron absorbs light and jumps from the lower set to the upper set across the gap (a d-d transition). The energy of light absorbed equals Δ, so the complex shows the complementary colour. A larger Δ means a shorter wavelength is absorbed.
